1. Field of the Invention
The present invention relates to a hydraulic closing unit such as used, for instance, in injection molding machines.
2. Related Art
The closing unit of an injection molding machine receives the injection mold. It carries out the movements necessary for the closing and opening of the injection mold and produces the forces necessary for the locking and opening of the injection mold. The main components of each closing unit are a stationary plate on the injection side (hereinafter referred to as the injection plate), a movable closure plate, as well as a locking device. One part of the injection mold is clamped on the stationary injection plate while the complementary part of the injection mold is clamped on the movable closure plate. By locking device there is to be understood the device which, upon the injection, produces the necessary closing force for keeping the injection mold closed. Both mechanical locking devices with lever mechanisms and hydraulic locking devices with hydraulic cylinders are known. The present invention relates to a closing unit with hydraulic locking.
From International Patent Application WO-A-93/16828, a closing unit for an injection molding machine is known in which the movable closure plate can be displaced between the stationary injection plate and an end plate which is also stationary by two displacement cylinders. The movable closure plate is provided with a central push rod which is guided in an annular insert in the stationary end plate. On this end plate a force cylinder is arranged which has a single-acting annular piston passed through axially by the push rod. A locking ring is guided in a cylindrical guide tube which is screwed onto the end plate. If the force cylinder is acted on by pressure, the annular piston of the force cylinder is advanced in the direction towards the movable closure plate. The front surface of the annular piston thereby comes against the facing end surface of the locking ring, the latter being pushed axially in its cylindrical guide tube in the direction towards the movable closure plate. In order to lock the locking ring on the push rod, the push rod has an outer toothing and the locking ring has a complementary inner toothing. The inner toothing and the outer toothing are divided into several rows of teeth by longitudinal grooves. The locking ring can be turned into first and second angular positions by means of a positioning cylinder. In the first angular position, the rows of teeth of the outer toothing can be passed through axially by longitudinal grooves of the inner toothing and the rows of teeth of the inner toothing by longitudinal grooves of the outer toothing, so that the push rod slides without substantial resistance through the locking ring. In the second angular position, the tooth of the outer toothing, on the other hand, can engage behind the teeth of the inner toothing. In this position, the locking ring is locked on the push rod. The annular piston of the force cylinder can exert a pressing force on the push rod via the locking ring and thus transmit the necessary closing force to the movable closure plate.
Upon the opening of the mold, the locking ring initially remains locked to the push rod. The two displacement cylinders produce a short rearward stroke in order to open the mold. By this short rearward stroke, the locking ring and the annular piston are moved backward simultaneously. Thereupon, the lock between the push rod and the locking ring is opened so that the push rod can slide through the locking ring when the closure plate is pulled back at high speed by the two displacement cylinders.
It may be emphasized that it would be desirable to produce the opening force for the opening of the injection mold also by the force cylinder. However, this is not possible in a closing unit in accordance with WO-A-93/16828 since the force cylinder is designed as a single-acting cylinder and furthermore it cannot be seen how a pulling force can be transferred in a simple manner from the piston via the turnable locking ring to the push rod.